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The Climate Impacts Group (CIG) translates global-scale climate forecasts
and conditions into regional-scale climate forecasts for Pacific
Northwest (PNW) resource managers and the general public. The
El Niño/Southern Oscillation (ENSO) is
the most important factor for seasonal forecasting, changing the odds
for different types of winter and spring weather (e.g. warmer/drier, cooler/wetter)
in the PNW. Another important climate variable for Pacific Northwest climate is the Pacific Decadal Oscillation (PDO). The climate outlook also provides the basis for natural resource
forecasts, including the CIG's annual streamflow forecasts.

What's Next for the Pacific Northwest?

September 2010
Updated 20 September 2010 (posted 21 September)

The climate outlook is reviewed monthly and updated as needed.

"La Niña [cold
ENSO] is expected to last through at least the
Northern Hemisphere winter 2010-11" (9 September NOAA
El
Niño Southern Oscillation (ENSO) diagnostic
discussion). The forecasts of the eventual strength of the episode
exhibit considerable range, from moderate to strong, and many of the models suggest
that the episode will peak during October through
January, slightly earlier than the typical ENSO
episode. The analysis of historical Pacific Northwest ENSO
variability suggests that the region will
experience cooler and wetter than normal
conditions in the coming months. A review of tropical Pacific observations and more recent
ENSO forecasts is found below.

The NOAA Climate Prediction Center (CPC) forecast for
October-November-December (OND) seasonal mean
precipitation is for a greater than 33
percent chance of above normal precipitation throughout the
Pacific Northwest, western Montana, northwest Wyoming, far
northern Nevada, and northern California. Chances of above normal
precipitation exceed 40% in north and central Idaho, all of Oregon
with the exception of the southeast corner, and all of Washington. The chances of above
normal precipitation exceed 50% in western Washington and
northwest Oregon. The prediction of increased chances of above
normal precipitation is consistent with storms bringing heavy precipitation
amounts and significant flooding to western Washington and
northwest Oregon.

The OND seasonal mean temperature forecast is for a greater than 33% chance of below normal temperatures to the
west of the Oregon and Washington Cascades, with the chances of the
same exceeding 40% in the coastal areas of the two states. The
remainder of the PNW is forecast to have an equal chance of below, near, and above normal
temperatures.

The seasonal forecasts should be interpreted as the tilting of odds towards
general categories of conditions, and should not be viewed as
a guarantee that the specified conditions will be
realized. In general,
seasonal precipitation forecasts only have skill during periods of significant
ENSO variability like the present (CPC
tools discussion).

For More Information

The Pacific Northwest, Montana, western Wyoming, northern Utah, northern
Nevada, and coastal, central, and northern California experienced below normal temperatures during the 30 days ending 19
September (1971-2000 mean, WRCC). The largest cold temperature
departures in the PNW, in the range -2 to -4 °F ( -1 to -2 °C) and
colder, were observed in smaller regions inland. The pattern of unseasonably cool
temperatures in the region began in mid-March (1 April through 19 September mean; temperature
timeseries for Lewiston, other cities, NCEP), and was interrupted by a
period of above normal temperatures inland from mid-July through early
August (24 July through 22 August mean).

August coastal sea surface temperatures (SSTs) continued
the colder than normal pattern that has been observed since April of
this year. SST departures were at least 1 °C colder than the 1985-97
mean from Vancouver Island to 30°N (Baja California), with larger
magnitude
temperature departures observed at the western end of the Straight of Juan de Fuca,
the central and southern Oregon coast, and for several regions along
the California coast. The mean northeast Pacific SST for the 30 days
ending 18 September depicts the colder than normal SSTs extending 5°
longitude offshore along the Washington coast, with the negative departures
extending southwest to Hawaii, and eastward from this line to the
coast (1982-96 mean, ESRL). Warm SST departures, in excess of
1°C, are observed between 25 and 45°N from Japan to the Gulf
of Alaska.

El
Niño/Southern Oscillation
(ENSO).
The 9 September NOAA El
Niño Southern Oscillation (ENSO) diagnostic
discussion summarized that
"La Niña [cold
ENSO] is expected to last through at least the
Northern Hemisphere winter 2010-11." The SST
departure from 1971-2000 averaged over 5°N-5°S, 170-120°W in the equatorial Pacific, in what is called
the "Niño 3.4" region, is employed as a key indicator
of ENSO variability. The ongoing cold ENSO, as measured by Nino 3.4 SST
departures, intensified slightly from July (-0.99
°C) to
August (-1.18 °C). The mean Nino 3.4 SST
departure for June-July-August was -0.90 °C.
The 3-month mean captures the seasonal evolution
of ENSO conditions and it is used in the NOAA
discussions to characterize the strength of the
ENSO episode.

NOAA
employs mechanistic and statistical models to forecast how ENSO will
evolve over the next several seasons. Mechanistic models solve
equations for ocean and atmosphere motions, and precipitation and radiative processes to
forecast the future from present conditions.
Statistical models, on the otherhand, are
constructed from observations of past climate, and
apply regression coefficients to present climate
conditions to forecast the future. Agreement of the forecasts from these two types of
models increases our confidence in the forecast.

The 3-month mean Nino 3.4 SST forecast from twenty-two ENSO models initialized with ocean and atmosphere data
through August are summarized by
the International Research Institute for
Climate and Society. Twenty of the models forecast mean October-November-December (OND) Nino 3.4 SST anomalies in
excess of -1.0 °C (moderate), twelve of the forecasts are
for Nino
3.4 < -1.5 °C (strong), and the mean forecast is -1.6 °C. There is considerable spread in the
SST predictions throughout the
forecast period. The mean forecast Nino 3.4 for
January-February-March and April-May-June 2011
are -1.2 and -0.5 °C, respectively.

Pacific Decadal
Oscillation (PDO).
The continued cold SST departures along
the west coast of North America and the
intensification of warm departures in the central
north Pacific, described above,
contributed to a drop in the PDO value from -1.05
to -1.27 standard deviations from July to August (1900-90
reference period, PDO
values). [For a normally distributed variable, only
32% of the values exceed one standard deviation in
magnitude.] August is the third consecutive month
of negative PDO values following positive PDO values that were observed last winter and
spring. ENSO variabilty is one
of the mechanisms that can produce changes in the
PDO, and the present cold ENSO can be expected to
contribute to negative PDO values in the coming seasons.

NOAA employs both
statistical and mechanistic models to forecast the
PDO and coastal ocean conditions. The statistical
linear
inverse model forecasts the PDO to be negative and of
diminishing magnitude through
the end of the forecast period in summer 2011.
The NCEP coupled forecast system (a mechanistic
ocean-atmosphere model) prediction issued on 20
September is
for a continuation of
the present pattern of cold SST anomalies along
the eastern boundary north of 40°N (Cape
Mendocino in California) and
warm anomalies in the central north Pacific
through March-April-May 2011 (20 September forecast, more recent
forecasts). The recent model forecasts for
next winter and spring have
been judged to not be skillful for the central and
southern California coast.